Page 147 - Uninterruptible Power Supplies

Static Uninterruptible Power Supplies
Static Uninterruptible Power Supplies 145

standard now is IEEE 519 (1992) which aims for 5 percent THD. In
Europe a similar standard is being considered EN 6100-3/2 which is in
a trial period at the moment.
Figure 5.3 illustrates the harmonics to be expected with 6- and 12-pulse
modules and the effect of running such units in parallel with some
phase-shifting transformers.
Figure 5.4 shows wavetraces using various six-pulse modules with
phase-shifting transformers, and an alternative fifth harmonic filter.
A fifth harmonic filter has financial advantages compared to a 12-pulse
rectifier but has some drawbacks. The filter is a tuned circuit, and vari-
ations in frequency which may occur when operating with a diesel gen-
erator set may be a problem. Standard diesel set specifications allow the
frequency to be controlled to 8 percent in which case it may result in
detuning of the filter, producing higher harmonics. However, most
installations use a close tolerance diesel generator set giving 1 or 2 per-
cent frequency regulation thus obviating the problem. Another cause for
concern is the dynamic response of modern diesel sets to a sudden load
change, which may also affect output frequency briefly. Low load condi-
tions may also affect the efficiency of the filter
The alternative is to use an active filter network, in essence a small
inverter circuit using a high switching speed (20 kHz) producing pulse
width modulated waveform. This is injected into the rectifier waveform

Number 6 Pulses 1 2 3 4
of Rectifiers 12 Pulses – 1 – 2
H3 1% 1% 1%
H5 41% 3% 3%
H7 11% 1% 1%
H11 7% 7% 1%
CURRENT H13 2.5% 2.5% 1%
HARMONIC H17 2.5% – –
DISTORTION H19 2% – –
H23 2.5% 2.5% 2.5%
H25 1% 1% 1%
H29 1% – –
H35 1% 1% –
H37 1% 1% –
THD* 43% 9.5% 5%

Figure 5.3

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